An X-ray CT apparatus is an apparatus which calculates an X-ray absorption coefficient on the basis of an X-ray transmission image (hereinafter, referred to as projection data) of an object obtained through scanning from a plurality of directions, and thus obtains a tomographic image (hereinafter, referred to as a reconstructed image) of the object, and is widely used in a medical field or a non-destructive inspection field.
A so-called integral X-ray detector is mounted in many current medical X-ray CT apparatuses, but, in recent years, an X-ray CT apparatus mounted with a photon counting type X-ray detector has been developed (for example, PTLs 1 and 2).
The X-ray detector includes an X-ray detection element having a detection layer of a semiconductor such as cadmium telluride (CdTe), and a reading circuit which classifies and obtains a digital signal for each energy range according to energy of incident X-ray photons. In this X-ray detector, if X-rays are incident to the X-ray detection element, first, electric charge corresponding to energy of X-ray photons is generated in the detection layer.
Next, the reading circuit reads the electric charge at a high speed at which each of the X-ray photons can be read, and classifies and counts the number of X-ray photons for each of several energy ranges according to energy of incident X-rays. In this case, the incident energy is identified by using an amount of generated electric charge.
The detection is similarly performed on each of a plurality of X-ray photons, the number of X-ray photons is counted in each energy range, and the counted number is converted into a digital signal. Through the measurement, projection data can be obtained for each energy range, and thus a reconstructed image can be obtained for each energy range by using the projection data. Energy information of an object can be obtained by using such projection data or a reconstructed image, and thus substance classifying performance can be improved.
In the photon counting type X-ray detector, in a case where a high dose of X-rays are incident per unit time, a plurality of X-ray photons may be incident during reading performed once. This phenomenon is called pile-up, a plurality of incident X-ray photons are counted as one, and energy thereof is detected as a wrong energy value. If the number of X-ray photons in each energy range is miscounted, accurate projection data cannot be obtained in each energy range. A reduction in quantitativeness of a CT value, deterioration in substance classifying performance, an artifact, and the like may occur in a reconstructed image created on the basis of the projection data.
In order to solve the pile-up problem, PTL 1 proposes a technique in which a preparation step of obtaining a correction coefficient for a counted number in each detection element is provided, and a measured counted number is corrected with the correction coefficient. PTL 2 discloses a technique in which, when incident energy is classified into respective energy ranges by using a predetermined threshold value, a threshold value for obtaining a correction value is set separately from an original threshold value, and a counted number in energy ranges defined by using this threshold value and a counted number in energy ranges defined by using the original threshold value are weight-added together so that a corrected counted number is obtained.